The present invention relates to a method for treating a liquid used for absorbing gaseous sulfur dioxide in the wet process for desulfurization of a combustion exhaust gas of the gypsum-by-producing type, and more in detail, the present invention relates to a method for removing chloride ions, which accumulate in the above-mentioned liquid used for absorbing gaseous sulfur dioxide, therefrom by the above-mentioned treatment.
Hitherto, the wet process for desulfurization of a combustion exhaust gas of the gypsum-by-producing type has been broadly carried out, wherein gaseous sulfur dioxide (hereinafter referred to as SO.sub.2) contained in a combustion exhaust gas, etc. is removed therefrom by a wet process, the thus absorbed So.sub.2 is neutralized with limestone or slaked lime, and by air-oxidizing the formed sulfite the gypsum is by-produced.
Usually, in the above-mentioned process for desulfurization, in order to prevent the scaling of gypsum and to improve the absorbability of SO.sub.2, a water-soluble sulfate is made coexistent in the liquid used for absorbing SO.sub.2, and as such a water-soluble sulfate, magnesium sulfate is usually used, because magnesium is contained in limestone. stone.
Furthermore, in the above-mentioned process for desulfurization, a carboxylic acid is added to the liquid used for absorbing SO.sub.2, thereby attempting the large improvement of SO.sub.2 -absorbability and the improvement of the reaction velocity between SO.sub.2 and limestone while utilizing the pH-buffer action of the carboxylic acid. At that time, a major part of the thus added carboxylic acid is present as carboxylate ions in the liquid used for absorbing SO.sub.2 (hereinafter referred to as the SO.sub.2 -absorbing liquid) and almost all the cations in the SO.sub.2 -absorbing liquid are Mg.sup.2+ derived from the limestone. Accordingly, the above-mentioned addition of a carboxylic acid is understood to make a magnesium carboxylate coexistent in the SO.sub.2 -absorbing liquid. As the above-mentioned carboxylic acid, succinic acid, glutaric, acid, adipic acid, fumaric acid, maleic acid, sulfosuccinic acid, etc. have been proposed.
Now, in the process for desulfurization of a combustion exhaust gas of the gypsum-by-producing type, a liquid which has been saturated by gypsum and has dissolved the above-mentioned components is used in circulation for absorption of SO.sub.2, and at that time, since gaseous hydrogen chloride which is contained in the combustion exhaust gas of coal, etc. is absorbed by the SO.sub.2 -absorbing liquid together with SO.sub.2, the thus absorbed chloride ions accumulate as magnesium chloride in the SO.sub.2 -absorbing liquid. Furthermore, the chlorides contained in the supply water which is supplied for the system of the above-mentioned process also become the origin of the accumulation of the chloride ion (hereinafter referred to as Cl.sup.-). Such as accumulation of Cl.sup.- is particularly remarkable in the case of using the SO.sub.2 -absorbing column of single scrubber (absorber) type not provided with a cooling column.
And, since the presence of Cl.sup.- of a high concentration in the SO.sub.2 -absorbing liquid becomes the origin of the corrosion of the apparatus of the system, it is necessary to remove Cl.sup.-, which accumulates in the circulated SO.sub.2 -absorbing liquid, therefrom.
Hitherto, as the method for removing Cl.sup.- accumulating in the SO.sub.2 -absorbing liquid therefrom, a method of discharging a part of the SO.sub.2 -absorbing liquid as a waste liquor from the system has been carried out. However, according to such a method, the above-mentioned magnesium sulfate and the magnesium carboxylate, which are useful components, are lost together with magnesium chloride from the SO.sub.2 -absorbing liquid. Accordingly, such a method is inadvisable.
In addition, according to the above-mentioned method, a SO.sub.2 -absorbing liquid of relatively low in concentration of Cl.sup.-, for instance, from 1 to 2% by weight, is discharged from the system, and the volume of the discharged liquid becomes large. Accordingly, the method is also economically unprofitable.
As another method for preventing the accumulation of Cl.sup.- in the SO.sub.2 -absorbing liquid, a method of electro dialysis while using an ion-exchange membrane has been known. This electrodialytic method by the use of the ion-exchange membrane is adopted as the concentration of sea water for a long time, and in the case of sea water, although Na.sup.+ and Cl.sup.- are the main components thereof, since Ca.sup.2+ and SO.sub.4.sup.2- are coexistent therewith, in the case of condensing such sea water, gypsum which is relatively low in solubility is deposited.
Accordingly, the scale-trouble due to gypsum is prevented by using a membrane selectively permeable to monovalent ions as the cation-exchange membrane and the anion-exchange membrane, respectively.
However, since the filtrate of the SO.sub.2 -absorbing liquid in the above-mentioned process for desulfurization has been saturated with gypsum premeditatedly, the deposition of gypsum is apt to occur, and moreover, since as the cation in the filtrate of the SO.sub.2 -absorbing liquid, Mg.sup.2+ which is an impurity of limestone is present in larger amount than monovalent cations, it is impossible to use a membrane selectively permeable to monovalent ions, in contrast with the case of electrodialysis of sea water.
Accordingly, in the filtrate of the SO.sub.2 -absorbing liquid, Ca.sup.2+ becomes much present in the concentrate of the electrodilytic cell to generate the gypsum scale on the ion-exchange membrane or within the membrane at the time of operation of electrodialysis. Namely, such a phenomenon makes the continuation of the operation of electrodialysis difficult. This is a large problem.
Furthermore, the application of the electrodialytic method to removal of Cl.sup.- from the liquid which has been saturated with gypsum has already been proposed, and among such methods, there has been effected a device for prevention of the scale-trouble due to gypsum.
For instance, in Japanese Patent Application Laying-Open (KOKAI) No. 55-81782/1980, the salts in the SO.sub.2 -absorbing liquid are divided into (1) the salts of monovalent cation - divalent anion series and (2) the salts of divalent cation - monovalent anion series by using an electrodialyzer having four dialyzing chambers comprising a usual pair of an anion-exchange membrane and a cation-exchange membrane combined with a pair of a membrane selectively permeable to monovalent anions and a membrane selectively permeable to monovalent cations as one unit.
However, since only the equivalent amount of Cl.sup.- to the amount of the monovalent cation can be removed by the above-mentioned method, such a method cannot be applied to removal of Cl.sup.- from the filtrate of the SO.sub.2 -absorbing liquid in the process for desulfurization of a combustion exhaust gas, in which Mg.sup.2+ is usually present in a large amount as the cation.
Furthermore, the deposition of gypsum is prevented by adjusting the pH of the SO.sub.2 -absorbing liquid to not higher than 4.5 (refer to Japanese Patent Application Laying-Open (KOKAI) No. 50-66481/1975) or by using a specified anion-exchange membrane (refer to Japanese Patent Application Laying-Open No. 53-123555/1978). However, any satisfactory result has not been obtained in the point of preventing the generation of the scale of gypsum by the above-mentioned methods.
As a result of the present inventors' studies for solving the problems of the above-mentioned publicly known methods for treating the SO.sub.2 -absorbing liquid, it has been found by the present inventors that in the wet process for desulfurization of a combustion exhaust gas of the gypsum-by-producing type, when an aqueous liquid which has been saturated with gypsum and has disolved at least magnesium sulfate and magnesium chloride is used as the SO.sub.2 -absorbing liquid and such an aqueous liquid is passed through an electrodialyzer having three dialyzing chambers comprising one sheet of cation-exchange membrane and two sheets of membrane selectively permeable to monovalent anions as one unit, the chloride ions (Cl.sup.-) accumulated in the above-mentioned circulated SO.sub.2 -absorbing liquid can be advantageously separated therefrom and at the same time, gypsum is not deposited on the ion-exchange membrane at the time of separating Cl.sup.-, and on the basis of the above-mentioned findings, the present inventors have accomplished the present invention.
Namely, the first object of the present invention is to provide a method for treating a SO.sub.2 -absorbing liquid in the wet process for desulfurization of a combustion exhaust gas of the gypsum-by-producing type, the method comprising selectively separating and removing Cl.sup.- as an aqueous solution of magnesium chloride of a high concentration from the SO.sub.2 -absorbing liquid which has been saturated with gypsum.
In the next place, the second object of the present invention is to provide a method for treating the above-mentioned SO.sub.2 -absorbing liquid without being accompanied by the deposition of gypsum in the above-mentioned wet process for desulfurization of a combustion exhaust gas of the gypsum-by-producing type.